Conventionally, there are pneumatic tires, in particular pneumatic passenger tires, including a tread surface having lateral grooves provided in its center region at predetermined intervals in the circumferential direction of the tires in order to enhance an effect of cutting water screen on road surfaces and removing it, the lateral grooves being inclined at a relatively small angle of, for example, about 10 to 45 degrees with respect to the tire circumferential direction. One of such pneumatic tires is arranged such that the inclined lateral grooves communicate at one ends with one circumferential groove and does not communicate at the other ends with another circumferential groove, the other ends being located within a rib.
This pneumatic tire can effectively drain water that has flowed into the lateral grooves by cutting water screen on road surfaces from the one ends of the lateral grooves to the one circumferential groove communicating therewith; however, since the other ends of the lateral grooves does not communicate with the another circumferential groove, the water flowed thereinto by cutting the water screen can not effectively be drained to the another circumferential groove; as a result, drainage characteristics have to be lowered.
To solve the above problem, there has been proposed a pneumatic tire having assistant grooves that extend from the center regions of the lateral grooves and communicate with the another circumferential groove (see Japanese Patent Application Kokai Publication No. 2004-17863, for example). The water flowing toward the other end side within the lateral grooves is drained through the assistant grooves, thereby improving drainage characteristics.
However, blocks that are small in contact area are defined by the another circumferential groove, lateral grooves, and assistant grooves adjacent the another circumferential groove; therefore, tread rigidity is locally lowered in the region where the blocks exist, thereby suffering from a problem in that the tread is subject to uneven wear.